Generally, in a turbine such as a gas turbine and a steam turbine, a seal device for reducing a leakage amount of a fluid that flows from the high-pressure side toward the low-pressure side is disposed between the outer peripheral surface of a rotor and a stator.
As a type of such seal device, known is a thin-plate seal structure having a plurality of thin plates (leafs) arranged in the circumferential direction of the rotor in a multiple-layered fashion, as disclosed in Patent Documents 1 to 3. The thin plates are normally in contact with the rotor while the turbine is stopped, but the thin-plate tip portions levitate from the rotor peripheral surface while the turbine is in operation, so that the thin plates are mainly in a non-contact state with the rotor. As compared to the labyrinth structure, the thin-plate seal structure is advantageous in that the leakage amount of fluid is small thanks to smaller clearance, as well as that abrasion of thin plates is less likely to occur thanks to the thin plates being in the non-contact state with the rotor more often in time series, thus having a longer seal lifetime.
Furthermore, with regard to the thin-plate seal structure, Patent Documents 1 to 3 disclose a configuration in which a low-pressure side plate and a high-pressure side plate are disposed on both sides, respectively, of the thin plates in the direction of the rotational axis of the rotor.
In Patent Document 1, the low-pressure side plate and the high-pressure side plate are used as guide plates in a direction in which a pressure of a fluid is applied. In Patent Documents 2 and 3, the low-pressure side plate and the high-pressure side plate are used mainly to form an appropriate static pressure distribution around the thin plates. That is, during operation of a turbine, the high-pressure side plate is in close contact with the side surfaces of the thin plates due to a pressure difference between the upstream side and the downstream side, so that the gap between the thin plates is closed in most section on the side of the thin-plate root portions (stator side). Thus, the fluid flows through the gap between the rotor peripheral surface and the rotor-side end portion of the high-pressure side plate, that is, from the side of the thin-plate tip portions (rotor side), into the gap between the thin plates. The fluid having flown into the gap between the thin plates from the side of the thin-plate tip portions forms an upward flow that flows from the thin-plate tip portions toward the root portions, and flows out from the gap between the rotor peripheral surface and the rotor-side end portion of the low-pressure side plate. The thin plates are disposed inclined with respect to the peripheral surface of the rotor. Thus, the thin-plate tip portions levitate due to the static pressure distribution formed by the upward flow of the fluid in the gap between the thin plates, and the thin plates separate from the rotor. Furthermore, besides the effect of the static pressure distribution, the dynamic pressure effect from rotation of the rotor also causes each thin plate to levitate.